1. Field of the Invention
This invention relates to methods of inhibiting mucin production using retinoid acid receptor antagonists, particularly retinoid acid receptor .alpha. (RAR.alpha.) selective antagonists.
2. Background Information
A. Retinoids
Retinoids are a class of compounds structurally related to vitamin A, comprising natural and synthetic compounds. A series of retinoids have been found clinically useful in the treatment of dermatological and oncological diseases. Retinoic acid and its retinoid analogs (9-cis RA, all-trans 3-4 didehydro RA, 4-oxo RA and retinol) are pleiotrophic regulatory compounds that modulate the structure and function of a wide variety of inflammatory, immune and structural cells. They are important regulators of epithelial cell proliferation, differentiation and morphogenesis in lung. Retinoids exert their biological effects through a series of nuclear receptors which are ligand inducible transcription factors belonging to the steroid/thyroid receptor superfamily. The retinoid receptors are classified into two families, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs), each consisting of three distinct subtypes (.alpha., .beta., and .gamma.). Each subtype of the RAR gene family encodes a variable number of isoforms arising from differential splicing of two primary RNA transcripts. All-trans retinoic acid (ATRA or RA) is the physiological hormone for the RAR's and binds with approximately equal affinity to all the three RAR subtypes. The RXR receptors do not bind to all-trans retinoic acid, but bind instead to the 9-cis isomer of retinoic acid.
Retinoids with retinoid receptor antagonistic activity (retinoid antagonists) are effective in counteracting many properties of retinoids with retinoid receptor agonist activity (retinoid agonists) such as inhibition of cell proliferation, induction of cell differentiation and inhibition of angiogenesis, Bollag, et al, Int. J. Cancer, 70:470-472 (1997) and in suppressing the toxic effects of retinoid agonists, Standeven, et al. Toxicol. Appl. Pharmacol., 138:169-175 (1996). Several classes of RAR antagonists have been reported in the literature including the RAR.alpha. selective antagonists described in M. Teng, et al. J. Med. Chem., 40:2445-2451 (1997); C. Apfel, et al. Proc. Nat. Acad. Sci. (USA), 89:7129-7133 (1992); L. Eyrolles, et al. J. Med. Chem., 37:1508-1517 (1994); H. Kagechika, et al. Biochem. Biopharm. Res. Commun., 231:243-248 (1997); and PCT publication WO 96/30009.
B. Mucin Secretion
Mucins are a family of glycoproteins secreted by the epithelial cells including those at the respiratory, gastrointestinal and female reproductive tracts. Mucins are responsible for the viscoelastic properties of mucus and at least eight mucin genes are known, D. J. Thornton, et al., J. Biol. Chem, 272:9561-9566 (1997). Many airway diseases such chronic bronchitis, chronic obstructive pulmonary disease, bronchietactis, asthma, cystic fibrosis and bacterial infections are characterized by mucin overproduction, E. Prescott, et al., Eur. Respir. J., 8:1333-1338 (1995); K. C. Kim, et al., Eur. Respir. J., 10:1438 (1997); D. Steiger, et al. Am. J. Respir. Cell Mol. Biol., 12:307-314 (1995). In particular, analysis of airway secretions has identified MUC5AC and MUC5B as the primary mucin constituents of the respiratory mucus gel. Mucociliary impairment caused by mucin hypersecretion leads to airway mucus plugging which promotes chronic infection, airflow obstruction and sometimes death. For example, chronic obstructive pulmonary disease (COPD), a disorder characterized by slowly progressive and irreversible airflow limitation is a major cause of death in developed countries. The respiratory degradation consists mainly of decreased luminal diameters due to airway wall thickening and increased mucus caused by goblet cell hyperplasia and hypersecretion. Historically, mucus hypersecretion has been treated in two ways: physical methods to increase clearance and mucolytic agents. Neither approach has yielded significant benefit to the patient or reduced mucus obstruction. Therefore, it would be desirable to have methods for reducing mucin production and treating the disorders associated with mucin hypersecretion.
The reported effects of retinoids on mucin expression are in conflict. Though certain investigators have reported that vitamin A (retinol) down-regulated expression of the MUC2 gene in tracheobronchial epithelial cells, G. An, et al., Am. Respir. Cell Mol. Biol., 10:546-551 (1994), others have reported that retinoid-replete cultures of normal human tracheobronchial epithelial cells produced an order of magnitude greater expression of MUC2 and MUC5AC mRNA than retinoid-depleted cells, K. Guzman, et al., Am. J. Physiol. 271 (Lung Cell Mol Physiol.15): L1023-L1028 (1996). Retinoic acid has been shown to be necessary for mucociliary differentiation of normal human tracheobronchial epithelial cells. In the absence of retinoic acid, the epithelium became squamous and mucin secretions decreased, Gray, et al., Am. Respir. Cell Mol. Biol., 14:104-112 (1996). However, the mechanism by which retinoic acid mediates its effects on tracheal epithelial cells is unknown, T. G. Christensen, et al., Am. J. Respir. Cell Mol. Biol., 9:287-294 (1993).